Arterial blood pressure (BP) is one of the most important vital signs and is widely used in clinical practice. Non-invasive arterial blood pressure (NIBP) is usually measured by slowly varying the pressure in a cuff that is wrapped around the upper arm of a subject. The BP is determined either by measuring sound distal from the cuff (the auscultatory method, based on Korotkoff sounds) or by measuring pressure pulsations in the cuff caused by volume pulsations of the arm and brachial artery and extracting features from the envelope of these pressure pulses (the oscillometric method). The oscillometric method is easily automated and is widely used.
The principle behind a typical oscillometric method is illustrated by FIG. 1, which shows a graph of cuff pressure 10, and a processed high pass filtered trace 12 of this cuff pressure, versus time. The left-hand y-axis shows pulse amplitude, the right-hand y-axis shows cuff pressure, and the x-axis shows time. To perform a NIBP measurement using the oscillometric method, first the cuff pressure 10 is ramped up until it is sufficiently larger than systolic blood pressure. After ramp up, the cuff is deflated (in FIG. 1 the deflation is done gradually, but step wise deflation is also possible). During the deflation, small oscillations in cuff pressure occur, caused by volume changes in the bladder of the cuff, which are in turn caused by volume changes in the brachial artery. The measured cuff pressure 10 is high pass filtered, and the resulting trace 12 shows the cuff pressure oscillations due to volume changes in the brachial artery. An envelope 14 of the oscillation amplitudes is determined. The maximum Amax of this pulse envelope 14 is taken as a reference point for determining the systolic 16 and diastolic pressure 15. The systolic pressure 16 is determined as the cuff pressure where the pressure oscillation is approximately 0.8 times the maximum amplitude Amax at a pressure higher than the pressure at the reference point. The diastolic pressure 15 is determined as the cuff pressure where the pressure oscillation is approximately 0.55 times the maximum amplitude Amax at a pressure lower than the pressure at the reference point. These ratios are based on empirical values (see, e.g., L A Geddes et. al., Annals of Biomedical Engineering 10 pp 271-280, 1982). The exact algorithms that are employed by manufacturers of blood pressure devices to determine systolic and diastolic pressures are usually trade secrets.
The typical monitor 20 used for acquiring oscillometric NIBP measurements is illustrated in FIG. 2. A pump 22, a pressure sensor 24, and a valve 26 are connected to a cuff 28 by tubing 30. A control unit 32 is connected to the pump 22 and the valve 26 to control the operation of those components, and is also connected to the pressure sensor 24 in order to receive the signal representing the pressure of the gas in the cuff 28 (the ‘pressure signal’). The control unit 32 runs the algorithm that controls the pump 22 and valve 26 and processes the pressure signal from the pressure sensor 24 to determine the BP measurement. During execution of the oscillometric method the pump 22 blows air into the cuff 28, thereby inflating it. The pressure sensor 24 measures the gas pressure in the system (and therefore the pressure of the gas in the cuff 28) and outputs a signal representing the pressure in the cuff 28 (referred to as the ‘pressure signal’). When a pressure larger than systolic pressure is reached, the pump 22 is disabled or switched off, the valve 26 is opened and slow (or step wise) deflation occurs, during which the cuff pressure is continuously measured and the measurements (pressure signal) stored. The pump 22 and valve 26 are controlled by a control unit 32, which also receives the cuff pressure measurements and calculates the pulse envelope and the systolic and diastolic pressure using these measurements. In practice the monitor 20 may comprise multiple sensors and valves for safety reasons.
The operation of the typical monitor 20 is often uncomfortable for the subject (and in some cases is painful), since the arm is compressed with an external pressure. In a clinical or hospital (or even home) setting where blood pressure measurements need to be obtained through the day and night, the taking of a blood pressure measurement by the monitor 20 will often disturb the sleep of the subject. NIBP monitors originally developed for high acuity subjects (e.g. those in an intensive care unit (ICU)) were optimized for accuracy and precision, but not the comfort of the subject.
In a home setting, it has been found that NIBP measurements have a relatively low acceptance by subjects (e.g. the subjects do not comply with the required measurement schedule or do not perform the measurements properly), which in some cases is due to the pain caused by the inflation of the cuff (which can relate to the duration that the cuff is inflated for and/or the peak pressure in the cuff), irritation of skin under the cuff (particularly on NIBP monitors that are continuously worn by a subject), haematomas, and disturbance of the sleep of the subject.
The comfort of the NIBP measurement can be improved in any or all of three areas: the total measurement time (where a reduction is desired), the maximum cuff pressure reached (where a lower maximum pressure is desired) and the integral of cuff pressure over time (where a smaller integral is desired). Of course, this increase in comfort should not come at the expense of the accuracy of the NIBP measurement beyond acceptable limits.
In addition to the types of monitor described above in which the BP is measured using envelope detection during deflation of the cuff (which can typically take around 45 seconds), monitors have been developed that can measure the BP while the cuff is being inflated. This can reduce the total measurement time (in some cases to around 20 seconds), since the deflation stage can be very quick once the BP measurement has been obtained, and therefore can result in a measurement that is more comfortable for the subject. However, currently available algorithms for measuring BP during the inflation of the cuff are not as accurate as conventional deflation-based algorithms, as inflation-based measurements are susceptible to measurement artefacts arising from movements by the subject or arrhythmias.
Therefore there is a need for an NIBP monitor and method of operating the same that measures the blood pressure during inflation of the cuff and that provides improved accuracy of the blood pressure measurement compared to conventional monitors.